OVER VIEW ON

METALLIC MATERIALS-2

PART 2 : HEAT TREATMENT

ALLOY SYSTEMS

STEELSALUMINUM ALLOYSTITANIUM ALLOYSNICKEL BASE SUPERALLOYS

STEELS

AnnealingNormalizingStress RelievingHardening and

TemperingSolution Treatment and

Aging

IMPORTANT EQUILIBRIUM PHASES IN

STEELS

Ferrite (α ) …Body Centered Cubic (BCC)

Cementite (Fe3C) … Orthorhombic

Austenite ( γ ) …Face Centered Cubic (FCC)

δ Ferrite …Body Centered Cubic (BCC)

BCC CRYSTAL MODEL

Packing Density- 68%

FCC CRYSTAL MODEL

Packing Density- 74%

BASIS FOR HEAT TREATMENT: Fe-C PHASE DIAGRAM

FEATURES

SOLUBILITY OF C IN

a Fe(BCC)-O.O2%

g Fe(FCC)-2.11%

EUTECTIC REACTION

11480C, 4.3%C

EUTECTOID REACTION

7270C, 0.77%C

OCTAHEDRAL VOID SPACE IN BCC

VOID SIZE IS 0.019 nm SIZE OF C ATOM IS 0.07

nm

OCTAHEDRAL VOID SPACE IN FCC

VOID SIZE IS 0.052 nm SIZE OF C ATOM IS 0.07

nm

SIZE OF VOID SPACE

ISOTHERMAL TRANSFORMATION CURVE

1. Heat the steel piece to the specified austenitizing temperature and hold so that the entire job achieves the specified temperature

2. Quench fast enough so as to avoid the knee to prevent formation of the high temperature transformation products (pearlite, ferrite and cementite)

HOW IS HARDENING DONE

EMERGENCE OF MARTENSITE FROM AUSTENITE

EFFECT OF CARBON

HARDNESS

WT% CARBON

Effect of Carbon on a) Hardness b) Ms

(a)

(b)

Diameter

Hard

ness

,R

c

Diameter

Hard

ness

,R

c

HARDENESS DISTRIBUTION

Hardness distribution in water quenched steels (a) SAE1045 and (b) SAE6140

(a) (b)

Depending upon requirement, appropriate tempering temperature is selected. A typical case is shown in the graph for 4340 STEEL. The tempering temperature depends upon the required strength and hardness after tempering. All these are tabulated and are available in ASTM literature for every steel.

Temper, C 204 260 315 371 426 482 538 594 650

(MPa)

(1725)

(1035)

(1380)

(690)

Tempering Temperature

Str

en

gt

h

EFFECT OF TEMPERING

These are softening processes used for producing steel with high ductility and low hardness. Though annealing is used in a very broad sense it has a distinct cycle.

Annealing involves heating the steel to elevated temperature, holding for a time dictated by section thickness and cooling in the furnace. The elevated temperature is in the range of 0-500c above a3 for hypo eutectoid steels and 0-500c above a1 (not acm, to avoid precipitation of pro-eutectoid cementite along grain boundaries) for hyper eutectoid steels Normalising involves heating the hypo eutectoid and hyper eutectoid steels above a1 and acm, respectively holding and air cooling.

ANNEALING AND NORMALISING TEMPERATURES

HEAT TREATMENT OF ALUMINIUM ALLOYS

Solid solution strengthened alloys Soaked in Furnace followed by air cooling

Precipitation hardened Solution treated and quenched( quench delay < 15

seconds) Aged natural (room temperature) or artificial (higher

temperature)

Al-Cu Phase Diagram

Al-Zn Phase Diagram

PRECIPITATION HARDENABLE ALUMINIUM ALLOY SYSTEMS

1) Solution Treatment- the alloy is heated above the solvus temperature and soaked there until a homogeneous solid solution (α) is produced. 2) Quenching is the second step where the solid α is rapidly cooled forming a supersaturated solid solution of αSS .3) Aging is the third step where the supersaturated α, αSS, is heated below the solvus temperature to produce a finely dispersed precipitate(θ). The formation of a finely dispersed precipitate in the alloy is the objective of the precipitation-hardening .

PRECIPITATION HARDENING PROCESS

STRENGTHENING PRECIPITATES IN DIFFERENT ALLOY SYSTEMS

Al- Cu systems and Al Cu Li systems Al2 Cu, Al2CuMg, Al2CuLi, Al3Li

Al –Mg-Si systems Al5Cu2Mg8Si6

Al-Zn-Mg, Al-Zn-Mg-Cu systems MgZn2 , Mg(ZnCuAl)2

Al-Mg Phase Diagram

Al-Mn Phase Diagram

SOLID SOLUTION STRENGTHENING

Solid solution strengthening is due to dissolved solute . The solute atmosphere interacts with moving dislocations impeding their motion.

WHY NO PRECIPITATES IN Al-Mg AND Al-Mn SYSTEMS?

Despite sloping solvus, Mg coming out of super saturated solution is extremely sluggish. Therefore strengthening is only by solid solution .Very slow cooling such as furnace cooling from annealing temperature brings out Mg in blocky form as Al3Mg2, reducing the strength.In higher Mg containing Al alloys (>4wt%) , these precipitates appear at grain boundary, reducing ductility and resistance to stress corrosion cracking.Post annealing cold work accentuates this problem.Mn in Al alloys is added below its high temperature solubility limit. Therefore no question of forming precipitates.Both Mg and Mn increase work hardening rate. Therefore strengthening is done by cold working(Al and Mg alloys < 3wt% Mg).

OTHER IMPORTANT Al ALLOYS Al-Si Alloys

Do not form any precipitatesWeak solid solution strengtheningSi improves fluidityTherefore used as sheets for brazing, Welding rods

and castings.

Al-Si-Mg alloys Si and Mg in proper proportion produce AlMg2Si precipitates. 2 Groups 1st Group- (Mg +Si ) 0.8-1.2 can be easily extruded and air

cooled. 2nd Group-(Mg +Si ) >1.4% develops high strength on aging

after ST + Quenching. Cu also added to enhance mechanical properties.

HEAT TREATMENT OF TITANIUM ALLOYS

Annealing Mill AnnealingNormal Annealing

Aging TreatmentSolution treatment and

quenchAgeing at elevated

temperature

HEAT TREATMENT

HEAT TREATMENT

DEVELOPMENT OF MICROSTRUCTURES

HEAT TREATMENT OF Ni BASE SUPERALLOYS

Solution Treatment Ageing Treatment

- Present at GBs- Observable only in sub-solvussoln. treated material(About 11% after 1105°C solutionising)

1000 – 10000 nms

Total volume fraction of ’ ~ 43%

- Present at GBs- Observable only in sub-solvussoln. treated material(About 11% after 1105°C solutionising)

1000 – 10000 nms

Total volume fraction of ’ ~ 43%

•HOMOGENIZATIONTo make the composition uniform•SOLUTION TREATMENTHeating to temperature above γ’ solvus and below incipient melting to take all the γ’ into solution, followed by quenching.Wrought alloys-1040-1230 CCast alloys-1180-1235 C•AGING1. Primary aging at

925 C to precipitate coarse γ’

2. secondary aging at 750 C to precipitate fine γ’

3. Tertiary aging at 700 C to precipitate very fine γ’ and to form M23C6 carbides.

HEAT TREATMENT DETAILS